/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} list1
 * @param {ListNode} list2
 * @return {ListNode}
 */
 var mergeTwoLists = function (l1, l2) {
    if (!l1) {
      return l2
    } else if (!l2) {
      return l1
    } else if (l1.val < l2.val) {
      l1.next = mergeTwoLists(l1.next, l2)
      return l1
    } else {
      l2.next = mergeTwoLists(l2.next, l1)
      return l2
    }
  };